1. Field of the Invention
This invention relates to pipelines wherein static electrical charges are present, and more particularly, to an apparatus and method for eliminating the build-up of static electrical charges in gas pipelines.
2. Description of the Background Art
The use of pipelines for transporting fluids, such as natural gas, is well known in the art. Such pipelines typically comprise a series of interconnected sections of pipe and fittings through which a fluid, typically consisting of liquid or gas, flows. The majority of modern gas distribution pipeline installations are fabricated from polyethylene pipe (hereinafter "PE pipe"). It has been estimated that there is about 350,000 miles of plastic pipe in gas distribution service in the U.S. and a substantial portion of that is PE piping.
One major problem with the use of PE pipe is relates to the electrical insulating properties of polyethylene. Specifically, gas flowing through a pipeline is capable of generating static electrical charges thereby creating an electrical potential in non-grounded, or electrically insulated sections of the pipeline, in a process known as triboelectrification. Specifically, when two relative electrical insulators rub against one another (e.g. gas and polyethylene), electrons are displaced from one molecule to another due to friction. Thus, when gas and particulate flow through PE pipe, electrons are displaced and have been found to accumulate on the inside wall of the pipe. Since the PE pipe has a high surface resistivity (electrical resistance), accumulating electrons are unable to dissipate to ground, and therefore generate varying voltage potentials between the inner and outer pipe surfaces. There does not appear to be a predictable pattern to charge accumulation. It has been found that voltages as high as 9000 volts may accumulate in PE pipe.
In pipelines fabricated from non-conductive materials the non-conductive nature of the piping materials prevents discharge by grounding. Thus, unless the static electrical charge is discharged by grounding, or otherwise neutralized, the voltage potential between the inner and outer pipe surfaces may reach levels sufficient for arcing to occur. The spark created when an electrostatic potential arcs represents a serious danger of explosion or fire, particularly when the fluid within the pipeline is flammable, or where the pipeline is in an otherwise hazardous environment. Furthermore, the electrostatic charge build-up in non-conductive pipe presents a hazardous problem to pipeline workers attempting to repair damaged pipelines, particularly those pipelines carrying flammable fluids such as natural gas. It has been found that the build-up of electrostatic charges can cause electrical shock to personnel that come in contact with a charged pipeline as well as triggering a fire and explosion when discharged by arcing.
In addition, the buildup of static electrical charges on the inside walls of PE pipe may cause arcing through the pipe wall resulting in the formation of a relatively small hole, commonly referred to as a "pinhole". As gas passes through buried PE pipe, voltages build up to a point exceeding the dielectric of the pipe causing arcing to occur between the inside and outside wall of the PE pipe as the electrons pass therethrough to ground. As is apparent, the presence of pinholes in natural gas pipe is highly undesirable. However, until technology is available to prevent the buildup of static electricity in the pipe the problems associated with arcing and pinholing persists.
The background art reveals a number of attempts directed to overcoming the dangers and problems associated with the presence of static electrical charges in pipelines. Most attempts, however, are limited to eliminating accumulated static electrical charges. Such attempts are not ideal, since it is difficult to determine whether static electrical charges have accumulated to problematic levels. For example, U.S. Pat. No. 3,943,273, issued to de Putter, discloses an electrically conducting plastic pipe system comprising a male plastic pipe part and a female plastic pipe part with conducting covering layers which are interconnected by means of a conducting sealing ring. The conducting components function to discharge static electrical charges by providing an electrically conductive path to ground.
U.S. Pat. No. 4,635,162, issued to McLaughlin, discloses conductive flanges for pipe fittings comprising a non-conductive flange including at least one strand of conductive yarn in conductive contact with the conduit and extending to the flange rim where it intersects one or several bolt holes. The flange is conductively attached to the conduit such that any static electrical charges are conducted by the yarn to the mounting. bolt, or bolts, disposed in the bolt holes. The bolts are grounded for discharging the potential by grounding the static charge thereby preventing sparking.
U.S. Pat. No. 5,173,333, issued to Tranbarger et al., discloses an apparatus and method for discharging static electricity on the internal surface of plastic pipe. Tranbarger's apparatus and method include providing a saddle for mounting on a section of pipe and a shaft having a cutter for penetrating the pipe upon engagement. The shaft also has a nozzle formed adjacent to the cutter for spraying an anti-static fluid into the plastic pipe for discharging or otherwise neutralizing static electricity. The devices and methods of the background art, however, possess a number of significant disadvantages. For example, the devices disclosed by de Putter and McLaughlin include complex pipe structures which are difficult to fabricate and thus have not realized great commercial success. Furthermore, those devices are not readily adaptable to discharge static electrical charges in existing pipelines without requiring the replacement of large sections of pipeline or pipe fittings. In addition, the device disclosed by Tranbarger et al. is affixed external to the pipeline and requires that the pipe be penetrated thereby breaching the integrity of the pipeline. Furthermore, since the Tranbarger et al. device is affixed external to the pipeline it is subject to damage, and, should the device become dislodged, the pipeline is subject to substantial leakage. Lastly, the Tranbarger device requires a complicated and unreliable mechanical device for injecting an anti-static fluid into the plastic pipe.
In U.S. Pat. No. 5,402,304, the present inventor has addressed the problem of neutralizing static charges which build up between sheets of paper stock, which patent is hereby incorporated herein by reference. That patent discloses an apparatus for eliminating static buildup in paper stock used within a paper handling machine, wherein an anti-static air enhancer cartridge is inserted in an airstream. The airstream passes across the enhancer cartridge where positively charged ions are released into the airstream. The positively charged ions are directed toward a supply of paper stock to neutralize static electrical charges associated therewith.
There still exists a need, however, for an apparatus and method for preventing the accumulation of static electrical charges on the inner surfaces of non-conductive gas pipe, which apparatus and method is adaptable to new as well as existing pipelines.